Traditionally automotive body structures have been predominantly made of steel sheet. However, more recently there has been a trend in the automotive industry to replace the heavier steel sheets with lighter aluminum sheets.
In order to be acceptable for automobile body sheet, however, an aluminum alloy must not only possess requisite characteristics of strength and corrosion resistance, for example, but also must exhibit good ductility and toughness.
Most of the aluminum alloys used in the automotive industry have been the aluminum-magnesium, or 5xxx series, and the aluminum-magnesium-silicon, or 6xxx series, alloys. While the automotive industry has seen the advent of high strength and ultra-high strength steels used for automobile construction, the 5xxx and 6xxx series alloys have reached their strength potential. Aluminum-zinc, or 7xxx series, alloys, however, offer significantly higher strengths than the 5xxx or 6xxx alloys thus making them excellent candidates to replace high strength steels. One of the disadvantages of 7xxx series alloys is the excessively long artificial aging time (up to 24 hours or longer) needed to achieve peak strengths. By contrast, the automotive industry is familiar with paint baking times which are typically less than 30 mins. In order to successfully implement the 7xxx series alloys into the automotive industry there is a need to reduce the artificial aging times.
Therefore, there is a need for improved methods to make 7xxx alloys which achieve desired properties of strength and ductility while reducing aging time, energy and cost.